Electropolishing metal



Jan. 27, 1959 D. R. ZAREMSK! 2,871,175

ELECTROPOLISHING METAL Filed June 25, 1957 INVENTOR. Donald R.Zoremsk|ATTORNE United States Patent ELECTROPOLISHIYG METAL Donald R. Zaremski,Cheswick, Pa., assignor to Allegheny Ludlum Steel Corporation,Brackenridge, Pa., :1 corporation of Pennsylvania Application June 25,1957, Serial No. 667,821

9 Claims. (Cl. 204-1405) This invention relates to the method ofelectropolishing metals and alloys and particularly metals of the classtitanium, titanium base alloys, zirconium and zirconium base alloys.

In the manufacture and processing of metals and metal products it isfrequently desirable to obtain a bright and lustrous surface. Sucha-surface is usually obtained by mechanical polishing and bufling or byelectrochemical treatment such as electroplating or electropolishing.Electropolishing has found wide application because it is economical andprovides a highly satisfactory bright and.

lustrous surface on most metals and alloys. This method of surfacetreatment is highly popular in modern manufacturing. Electropolishingconsists of immersing the metal object to be polished as an anode in anelectrolyte and passing a current through the electrolyte between theobject being polished and another object or cathode. The electrolytesgenerally employed are aqueous acid solutions. Some metals and alloysare difficult to electropolish because of their inability to passcurrent as an anode in an aqueous electrolyte. This is particularly trueof the metals titanium, titanium base alloys, zirconium and zirconiumbase alloys. Although it has been possible to electropolish these metalson a laboratory scale with aqueous type electrolytes, these methods haveproved to be impractical when applied commercially It has now been foundthat metalsand alloys, and par ticularly titanium, titanium base alloys,zirconium and zirconium base alloys may be electropolished in moltencaustic salt baths without employing the usual impressed electricalcurrents.

It is therefore an object of the present invention to provide a methodof electropolishing the surface of metals and alloys.

Another object of this invention is to provide for the electropolishingof metals that cannot be satisfactorily electropolished in aqueouselectrolytes.

A more specific object of the present invention is to provide a methodof electropolishing metals selected from the group consisting oftitanium, titanium base alloys,

zirconium and zirconium base alloys.

Other objects and advantageous features may be best understood bythe'following description when taken in conjunction with theaccompanying drawing in which:

Figure 1 is a view, partially in section, of an electropolishing cellsuch as is employed in practicing this invention, and

Fig. 2 is a view in section of the electropolishing cell takenalong thelines II.-II of Fig. 1.

' In practicing this invention the metal to be polished is immersed in amolten caustic salt bath and is coupled to a metal also in directcontactwith the bath that is more noble in the caustic salt bath than the metalbeing treated. Reactive metals of the class titanium, titanium basealloys, zirconium and zirconium base alloys, and particularly suchmetals in the form of sheet'products, are benefited by being connectedto another metal 2,871,175 Patented Jan. 27, 1959 object in the bath sothat the' reactive metal is provided with an electrical potential and isanodic with respect to the other metal object in the bath. In practicingthe invention, an external electrical connection is made between themetal being treated and another metal that is chemicallyelectronegative, in the molten caustic salt bath, to the metal beingtreated. Thus there i created a simple electrolytic cell in which themolten caustic salt is an electrolyte, the metal object being treated isan anode, and a second metal object is a cathode. The use of impressedcurrents or providing electrical current from an outside source isneither necessary nor desirable. It is preferred to employ theelectrochemical reaction obtained only when a second metal that iselectronegative in the caustic salt bath to the metal being treated isemployed in order to provide a cell type reaction.

Referring to the drawings, this invention is illustrated by reference tothe immersion of a sheet 11 of titanium or the like and a sheet 12 in abath 13 of molten caustic salt contained in a steel tank 14. The sheet12 may be of any metal that is electronegative with respect to the metalof sheet 11 in the caustic salt bath 13. In practice the material of theconstruction of the tank 14 is not critical and may be any metal ormaterial satisfactory for holding a molten caustic bath under theconditions employed.

The caustic bath 13 is maintained in a molten state. It should be notedthat the upper edges of sheets 11 and 12 extend above the upper level 15of salt bath 13 and that in this area of sheets 11 and 12, representedin Fig. 1 as 16, there is an external electrical connection 17 which inthis embodiment is a wire cable. Wire connection 17 may be clamped,welded or otherwise attached to sheets 11 and 12 in any conventionalmanner so long as an electrical coupling is made. In the presentembodiment there is shown a substantially nonconductive insulatingmaterial 18 which may be made of any nonelectrical conductive substancewhich is used to prevent either of sheets 11 or 12 from touching 'thesteel tank 14 to avoid electrical currents caused by potentialdifferences between the metal of sheets 11 and 12 and the steel of thetanks from flowing between the sheets and the tank. However, should thetank be made of a substantially nonconductive material, such insulationas is provided by 18 would be unnecessary.

Reference is particularly directed to Fig. 2 wherein is shown theelectric cell type of reaction employed. Sheet 11 is anodic in the saltbath 13 with respect to sheet 12 which functions as the cathode. Sincethe sheets are provided with an external electrical connection 17,

l a natural flow of electrons and ions takes place between the twosheets. The anode 11 dissolves and titanium ions leave the surface ofthe metal, leaving behind electrons. The titanium ions thus take on apositive valence. These particles then tend to migrate towards thecathode or sheet 12. The negatively charged particles or electrons onanode 11 flow through the external electrical connection 17 to thecathode sheet 12, thus increasing the attraction of the cathode for thedissolved positively charged ions of titanium. It is to be noted thatthe flow of electrons through the external connection is in a directionopposite to the conventionally assumed flow of electrical current.

It can bereadily observed from the above that the anode dissolves andlooses metal from its surface. By such a process the anode surface isevenly attacked in the molten caustic bath, leaving a bright andlustrous surface.

The caustic bath employed may be any of the known com'me'rcialsalt bathswhich usually have an alkali metal base and arepreferably heated, and inuse are preferably maintained at a temperature in the neighborhood of875 F. to 975 P. so that the salt bath is in a molten state and willreadily act as an electrolyte for the electropolishing of the anodicmetal. The compositions of these baths consist essentially of alkalimetal hydroxides and preferably sodium and potassium hydroxides, plusminor additions of inhibitors, oxidizing agents, etc. While manyvariations in the composition of the caustic bath are possible, atypical example of a suitable caustic bath, is that of the commerciallyknown Virgo salts having about 88.55% sodium hydroxide, about 7.66%sodium nitrate, 1.75% sodium chloride and 0.53% sodium carbonate withtraces of manganese dioxide present. The Kolene salts formed of alkalimetal hydroxides, nitrates and chlorides as de scribed in Patent No.2,458,661 are also effective in carrying out the metal treatment of thepresent invention.

The magnitude of the current necessary to carry out the electrochemicaltreatment of the present invention is relatively small. The cathode mustbe a metal that is electronegative to the metal or alloy being treated.For example, it is preferable to use iron or steel as the cathode whentreating titanium. Other metals that are cathodic to titanium in moltencaustic Virgo are aluminum, molybdenum, chromium and, of course, alloysof these metals. The electrical potential created by employing the celltype of reaction provides the necessary low amperage currents thateffectively electropclish the anodic metals. In electropolishingtitanium the potential that exists between titanium and steel electrodesin Virgo baths generates sufficient electrical current to effectivelyelectropolish the surface of the titanium. As shown by Table 1 below,the current density in such a cell drops off rapidly relative to time.When the electrical connection is first made, a current density ofslightly less than 2 amperes per square inch exists; however, after 10minutes the current has decreased to about ampere per square inch. Thisreduction in current density does not materially reduce theelectropolishing action of the bath. Some electropolishing Will beaccomplished so long as the metal being treated is maintained anodic,regardless of how low the current density may become.

When reactive metals such astitanium, titanium base alloys, zirconiumand zirconium base alloys are immersed in molten caustic salt baths suchas Virgo and are insulated from any electrical connection or cathodicmember, chemical and cell type electrochemical attack frequently takesplace. The reactive metal does not dissolve evenly and thus the metalsurface is subjected to pitting and cavitation. This is believed to becaused by the presence of hydrogen in the caustic bath which replacedtitanium, causing gas evolution in the vicinity of the metal beingtreated. The dangers of hydrogen are eliminated when the metal or alloybeing electropolished is made anodic to one or more objects in themolten salt bath since hydrogen tends to migrate to the cathode. Whenthe reactive metals titanium, zirconium or their alloys are immersed bythemselves in a molten caustic salt bath, hydrogen gas evolution may bevisibly observed. However, when such metals are made anodic with respectto other objects in the bath there is no 'gas evolution. The reaction isdefinitely electrochemical and the anodes dissolve evenly leaving asmooth, bright and lustrous surface.

The time required to effect electropolishing does not appear to becritical. However, excellent results have been obtained on titanium whenimmersed in the caustic salt bath while employing steel as cathodes fora time of from 5 to 7 minutes.

Excellent results were obtained on titanium samples as follows: 2" x 3sample plates were constructed of various steels, commercial gradetitanium and titanium alloys. One sample of each type of titanium alloyand one sample of titanium were electrically connected at one end to alike sample of plain carbon steel. Another sample of each type oftitanium alloy and one sample of commercial grade titanium wereelectrically connected in a like manner to a Type 316 stainless steelsample. In each case the electrical connection was made external of thebath and each pair of samples was immersed in molten Virgo salts withthe upper edges of the samples and the electrical connections free fromthe molten salt. The titanium grades tested were Ti 75A (commercialgrade titanium), 6% aluminum4% vanadiurn-Ti alloy and 2% iron2% chromiumand 2% molybdenum-Ti alloy. The temperature of the Virgo wasapproximately 925 F. The time in solution was from 5 to 7 minutes. Uponremoval from the electrolyte the titanium alloy samples were waterquenched and then given a hot nitric acid dip to remove the adheringVirgo. Smooth, bright and lustrous electropolished surfaces wereobtained on all the titanium and titanium alloy samples.

Similar samples of 2% iron-2% chromium and 2% molybdenum titanium alloysheet were immersed in molten Virgo (925 F.) while being completelyinsulated from all other metal parts in contact with the Virgo bath. Thesamples were removed from the bath after a period of about 10 minutesand immersed in a nitric acid bath to remove the Virgo. All the samplesexhibited badly pitted surfaces with the pits aligned in a direction atright angles to the plane of the bath surface. There was no surfacebrightening.

Similar samples of titanium Type Ti 75A (commercial grade titanium) weretested in Virgo while employing carbon steel and stainless steel Type316 as the cathodes. The current densities were measured on these testsfrom the time of immersion and for 10 minutes. The results are asfollows:

Table I Current densities mp Time Titaniurn/ Titanlum/ Virgo/carbonVirgo/stainsteel less 316 1. 669 1.785 1. 234 0. 492 0. 686 O. 481 0.503 0. 400 0. 457 (I. 385 0. 407 0. 308 O. 389 l). 283 0. 366 0.277 0.357 O. 262 0. 246 0. 288 O. 222

As can be seen in the above table the current density drops off rapidlyin respect to time. The current drop is believed to be caused bycathodic polarization. Such drop in current does not materially reducethe electrochemical eifect of the treatment.

The process of the present invention may be applied to any metal objectand is particularly applicable to objects made from reactive metals suchas titanium, zirconium and alloys thereof. The treatment is particularlyadvantageous when used in connection with materials with fiat surfacessuch as plates and sheets. The present application has described thebatch type treatment such as is applicable to. sheets or plates;however, it is to be understood that modifications may be employed forthe continuous treatment of strip material passing through a causticbath. Strip material made of titanium,

zirconium or their alloys and which are continuously pickled in causticsalt baths may be treated by conducting a similar steel strip throughthe bath simultaneously with the reactive metal strip while maintaininga spaced relation between the two metals and providing an electricalconnection therebetween. Where steel containers hold the molten causticbath, it may be desirable to pro vide an electrical connection betweenthe metal being treated and the steel container, thus making thecontainer a cathode.

Although this invention has been described with reference to thepreferred practices and by specific illustration, it is to be understoodthat the invention is susceptible to variation and extension and thatthe scope of the specification and claims is'to be interpreted broadly.

Iclaim:

l. The method of electropolishing the surface of an object made of ametal selected from the group consisting of titanium, titanium basealloys, zirconium and zirconium base alloys which comprises, immersingsaid object in a molten bath that consists essentially of an alkalimetal hydroxide, providing at least one other metallic object in saidbath that is more noble in said bath to titanium, zirconium and alloysthereof and-providing an electric coupling between said objects so as toprovide an electric current.

2. The method of electropolishing the surface of an object made from ametal selected from the group consisting of titanium and titanium basealloys, immersing said object in a molten bath that consists essentiallyof an alkali metal hydroxide, providing at least one other metallicobject in said bath which is more noble in said bath to titanium andalloys thereof and providing an electrical coupling between said objectsso as to provide a flow of electrons.

3. The method of electropolishing the surface of an object made from ametal selected from the group consisting of zirconium and zirconium basealloys, immersing said object in a molten bath that consists essentiallyof an alkali metal hydroxide, providing at least one other metallicobject in said bath which is more noble in said bath to zirconium andalloys thereof and providing an electrical coupling between said objectsso as to provide a flow of electrons.

4. The method of electropolishing a reactive metal of the class oftitanium and titanium base alloys which comprises, immersing saidreactive metal in a molten salt bath, the temperature of which is withinthe range of from 875 F. to 975 F. and the composition of which consistsessentially of sodium hydroxide, providing an electric coupling betweensaid reactive metal as the anode and at least one other object in saidbath that is more noble than said reactive metal, so as to provide anelectric current between said reactive metal and said object of fromapproximately 2 to A amperes per square inch.

5. The method of electropolishing a reactive metal of the class titaniumand titanium base alloys which comprises, immersing said reactive metalin a molten salt bath, the temperature of which is within the range offrom 875 F. to 975 F. and the composition of which consists essentiallyof sodium hydroxide, providing a terial in a molten salt bath thatconsists essentially of an alkali metal hydroxide, providing at leastone other metallic object in said bath that is more noble in said bathto said sheet material, and providing an electric coupling external ofsaid bath between said sheet material and said object so as to providean electric current.

7. The method of electropolishing sheet material formed from refractorymetal selected from the class of titanium, titanium base alloys,zirconium and zirconium base alloys which comprises; partially immersingsaid sheet material in a molten salt bath that consists essentially ofan alkali metal hydroxide, partially immersing a like sheet in said bathof a metal that is more noble in said bath to said refractory metalsheet, maintaining said sheets in a spaced relationship to one anotherand providing an electric coupling external of said bath between saidsheets so as to provide an electric current passing between said sheets,the refractory metal sheet being the anode.

8. The method of electropolishing a reactive metal selected from thegroup consisting of titanium, titanium base alloys, zirconium andzirconium base alloys which comprises, immersing said object in a moltenbath that consists essentially of at least one alkali metal hydroxideselected from the group consisting of sodium hydroxide and potassiumhydroxide, providing at least one other metallic object in said baththat is more noble in said bath to titanium, titanium base alloys,zirconium and zirconium base alloys, and providing an electric couplingbetween said objects so as to provide an electric current.

9. The method of electropolishing a reactive metal of' the classtitanium and titanium base alloys, which comprises, immersing saidreactive metal in a molten salt bath, the composition of which consistsessentially of sodium hydroxide, providing at least one second metal tosaid bath of the class, aluminum, molybdenum, chromium, iron and alloysof aluminum, molybdenum, chromium and iron, providing an electriccoupling between said reactive metal and second metal so as to providean electric current.

References Cited in the file of this patent Metallurgia, vol. 49, No.295, May 1954, page 256, by Durkin.

1. THE METHOD OF ELECTROPOLISHING THE SURFACE OF AN OBJECT MADE OF AMETAL SELECTED FROM THE GROUP CONSISTING OF TITANIUM, TITANIUM BASEALLOYS, ZIRCONIUM AND ZIRCONIUM BASE ALLOYS WHICH COMPRISES, IMMERSINGSAID OBJECT IN A MOLTEN BATH THAT CONSISTS ESSENTIALLY OF AN ALKALIMETAL HYDROXIDE, PROVIDING AT LEAST ONE OTHER METALLIC OBJECT IN SAIDBATH THAT IS MORE NOBLE IN SAID BATH TO TITANIUM, ZIRCONIUM AND ALLOYSTHEREOF AND PROVIDING AN ELECTRIC COUPLING BETWEEN SAID OBJECTS SO AS TOPROVIDE AN ELECTRIC CURRENT.